A “balun” device (from the acronym of the English terms BALanced/UNbalanced) is a transformer connected between a balanced source or load and an unbalanced source or load. As it is known, a balanced signal line comprises two conductors for the signal that are passed through by equal currents in opposite directions. An unbalanced signal line comprises only one conductor passed through by a current and the common ground potential GND represents the return path for such current.
The electronic devices for radio-frequency (RF) applications without wires, or wireless applications, generally comprise respective input/output terminals for signals of the balanced type, i.e., input/output terminals of the differential type, to minimize the effects of substrate inductances and to improve the common mode rejection. Such electronic devices with input/output differential terminals comprise, for example, mixers, modulators, and voltage-controlled oscillators, or VCO.
As it is known, on the balanced output terminals of such devices, differential signals exist, which have to be mutually combined to generate an output signal of the single-ended type to be supplied outwardly. To this purpose, the balun device is suitable for connecting such balanced output terminals to a single unbalanced output terminal in order to convert the differential output signals into an output signal of the single-ended type. Similarly, the balun is suitable for converting an unbalanced or single-ended input signal into differential input signals for the above-mentioned electronic devices.
In the realization of printed circuit boards or PCB for RF applications, it is known to manufacture a balun transformer circuit including a first portion that is manufactured by means of a metal track that is printed on one of the planar surfaces of the substrate board of the circuit. On the same planar surface of the board, such known balun further comprises a respective second transformer portion, generally manufactured by a coaxial cable, connected to the first printed portion. In particular, the printed metal track is shaped so as to comprise a first and a second terminal end connected, for example, by welding, to corresponding terminal ends of the coaxial cable.
Such known balun transformer produced on a board of a printed circuit is not free from defects.
In fact, the Applicant has verified that an inaccurate shaping of the coaxial cable before its securing on the board, or an imprecision in carrying out the welding that connects both the core and the cladding of the coaxial cable to the first and the second terminal end of the printed metal track can introduce parasitic effects (for example, undesired phase displacements) that alter the converted signal. Furthermore, in radio-frequency applications, for example, at frequencies of the order of about 1 GHz, such parasitic effects are mostly apparent, such as to compromise the predictability of the signals converted by the balun.